The present invention relates to a non-linear feedback controller for an internal combustion engine which is effective in stably controlling the rotational speed of the internal combustion engine or in controlling the engine speed so as to converge to a target rotational speed with superior follow-up characteristics by using a parameter which is determined on the basis of a dynamic physical model of the internal combustion engine.
A technique has heretofore been known by which a dynamic model of an internal combustion engine is constructed on the basis of a control theory that takes into consideration the internal state of the engine, and a variable which is to be input to the internal combustion engine to be controlled is determined by estimating a dynamic behavior of the engine on the basis of state variables which represents the internal state of the engine. One example of such technique has been proposed as a "Method of Simultaneously Controlling Idling Speed and Air-Fuel Ratio in Internal Combustion Engine" (Japanese Patent Laid-Open No. 59-7751). More specifically, state variables of appropriate order which represent the dynamic internal state of an internal combustion engine are estimated on the basis of a dynamic model of the engine in which control inputs include the air quantity, the fuel supply quantity and the ignition timing or the exhaust recirculating quantity; control outputs include the idling speed and the air-fuel ratio; and a multi-variable control is effected with the above-described control inputs and outputs. In particular, when there is a change in the dynamics of the engine, the dynamic model and control gain are changed for appropriate other ones, thereby simultaneously effecting optimal control of the rotational speed and air-fuel ratio during idling of the engine in accordance with the dynamics of the engine, and thus realizing even more stable idling.
In this prior art, the state variables are not required to correspond to various kinds of physical quantities which represent the actual internal state, but these variables generally simulate the engine. Further, according to the above-described prior art, a parameter (e.g., cooling water temperature) is determined in order to detect the fact that there is a change in the dynamics of the engine, and dynamic models are stored in advance in correspondence with various values of the parameter, so that the dynamic models and control gains are changed from one to another in accordance with the value of the parameter.
In regard to a complicated control object such as an internal combustion engine, it has heretofore been difficult to obtain theoretically a precise dynamic model of such a complex control object, and therefore has been necessary to determine the model experimentally in some way. Accordingly, in the above-described prior art, the relationship between each control input and control output is expressed by a transfer function matrix which has been obtained in the vicinity of a certain reference set value and then linearly extrapolated, and the transfer function matrix is determined by the so-called system identification technique, thereby constructing a dynamic model of the internal combustion engine. However, the dynamic model thus determined expresses the behavior of the internal combustion engine for perturbations only in the vicinity of a specific operating condition, that is, near the above-described reference set value, and it is a model which does not necessarily have physical meanings. Therefore, in general, the dynamic model does not effectively match the internal combustion engine that is the control object.
Thus, when the operating condition of the internal combustion engine changes over a wide range, that is, when the engine is frequently run in transient states at the time, for example, of cold starting, warming-up, idling after the completion of warming-up, heavy-load operation during starting or acceleration, and light-load operation during constant-speed running, the actual behavior of the internal combustion engine deviates from the predetermined dynamic model to a substantial extent, resulting in a reduced degree of control accuracy. Accordingly, it has heretofore been difficult to effect satisfactory feedback control.
To overcome such a problem, the above-described prior art is arranged such that a plurality of linear models are determined, corresponding to various operating conditions of an internal combustion engine, and these linear models are changed from one to another to effect accurate control. The predetermination of a plurality of linear models complicates the control law, however, and leads to poorer control response and follow-up characteristics. Moreover, when control is effected at the boundary region between linear models, it is impossible to predict what kind of phenomenon will occur.
To cope with the above-described problem, the applicant of this application has already proposed, for example, "Feedback Control Method for Internal Combustion Engine" (Japanese Patent Application No. 61-220687). In this proposed method, a control quantity for feedback control is determined on the basis of formula models obtained by making discrete samples of a dynamic physical model of an internal combustion engine by means of sampling effected every predetermined crank angle, the dynamic physical model being constructed using at least a quantity which is equivalent to the pressure of intake air sucked into the engine and a quantity which is equivalent to the rotational speed of the engine. This eliminates the need to change the control law even when the operating condition of the internal combustion engine changes over a wide range. In this improved art, the dynamic physical model of the internal combustion engine is based on the assumption that the quantity of air passing through the throttle valve is independent of the intake pressure and proportional only to the opening area of the intake passage.
Subsequent research has determined that when the intake pressure is equal to or less than the critical pressure (for example, if the intake pressure P is 53.7 KPa or less when the atmospheric pressure PO (at the upstream side of the throttle valve) is 101.32 KPa), that is, when the engine is run under a relatively light load with a relatively small throttle valve opening, the flow velocity of the intake air when passing the vicinity of the throttle valve has a constant value which is substantially equal to the velocity of sound, and the above-described assumption is valid. However, when the intake pressure exceeds the critical pressure, that is, when the engine is run under a relatively heavy load with a relatively large throttle valve opening, the flow velocity of the intake air passing the vicinity of the throttle valve changes under the effect of the intake pressure, and the above-described assumption is not necessarily valid. Accordingly, in the case where the behavior of the internal combustion engine expressed by the dynamic physical model differs from the actual dynamic behavior of the engine during a heavy-load running at the time, for example, of starting or acceleration, the degree of control accuracy may fall, which means that the abovedescribed improved art is still unsatisfactory.